Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
The use of wireless communication devices (WCDs) is becoming increasingly prevalent and popular in modern life, and more people than ever are using mobile WCDs to connect to wireless communication systems. To provide wireless communication service to WCDs (also often referred to as smart phones, client devices, user equipment, mobile stations, subscriber devices, access terminals, and the like), a wireless service provider typically operates and manages one or more wireless communication systems. In a typical wireless communication system, a radio access network (RAN) includes one or more base stations that radiate to define one or more wireless coverage areas such as cells and cell sectors in which suitably equipped WCDs can operate.
In practice, the RAN and its served WCDs may be arranged to communicate with each other according to an agreed air interface protocol that defines a mechanism for use of communication resources to support wireless exchange of voice, data, multimedia, and/or other content. Examples of such protocols include CDMA (e.g., 1xRTT or 1xEV-DO), LTE, WiMAX, IDEN, GSM, WIFI, HSDPA, among others now known or later developed. The air interface in each coverage area is typically divided into a plurality of channels for carrying communications between the WCDs and the wireless communication system. For example, the air interface may define a forward-link (or “downlink”) for carrying communications from the RAN to the WCDs and a reverse-link (or “uplink”) for carrying communications from the WCDs to the RAN. In various implementations, these links may be defined on one or more carrier frequencies or blocks of frequencies. Furthermore, on each link, various channels may be defined through techniques such as time division multiplexing, code division multiplexing, frequency division multiplexing, and the like.
By way of example, the forward-link may define a pilot channel for carrying a pilot signal usable by WCDs to detect and evaluate coverage, a paging channel for carrying page messages to particular WCDs, other overhead channels for carrying system parameter information and the like, and a number of traffic channels for carrying bearer traffic (e.g., call traffic) to WCDs. The reverse-link, on the other hand, may define an access channel for carrying messages from WCDs to facilitate registration, responding to pages, originating calls and the like, and a number of traffic channels for carrying bearer traffic from WCDs to the RAN. Depending on the protocol, the channel definitions may differ and other names for the channels may be used.
In practice, the reverse link access channel may define a number of containers, such as timeslots or resource elements, for carrying access channel messages from WCDs to the RAN. WCDs may then be arranged to transmit access channel messages called “access probes” in these containers. For instance, when a WCD first enters into wireless coverage and at other times, the WCD may transmit a radio access registration message in an access probe to the RAN, to notify the RAN where the WCD is located. Further, when the WCD seeks to originate a call (e.g., a voice call and/or data session), the WCD may transmit an origination message in an access probe to the RAN to trigger setup of the call. Further still, when a WCD receives a page message from the RAN on the forward link paging channel, the WCD may transmit a page response message in an access probe to the RAN. Moreover, the WCD may be arranged to transmit certain user data such as short message service (SMS) messages as data bursts in access probes to the RAN, to avoid the need to set up an air interface traffic channel for such communication.
To help determine which wireless communication systems a given WCD may operate in, WCDs typically store a “preferred roaming list” (PRL), which includes a prioritized listing of wireless communications systems that the WCD may access. In practice, each available wireless communication system may emit a pilot signal (or the like), via the pilot channel, that uniquely identifies the wireless communication system. For each wireless communication system listed in the PRL, the PRL may then specify corresponding access data, such as radio-wave frequency and system/network identification code. Using the specified data, the WCD may then search for a wireless communication system (i.e., scan the airwaves in search of a system) in the order the wireless communication systems are listed in its PRL and may then register with the highest priority system it finds.
As a general matter, a PRL can take any of a variety of forms. By way of example, a rudimentary PRL may take the form of a simple flat file listing or data table in which each entry corresponds with a respective system and indicates a radio frequency and system/network identifier for the system. Using such a PRL, a WCD may repeatedly sequence through the listed entries, for each entry scanning the indicated frequency in search of a pilot carrying the indicated identifier, and the WCD may register with the first (i.e., highest priority) system that it finds.
More typically, however, a PRL will include a separate “acquisition table” and “system table.” The acquisition table may list frequencies to be scanned for allowed systems, and the system table may then provide a priority-ordered listing of allowed systems, specifying for each system an associated system identifier and/or network identifier. Under CDMA, for instance, each system table entry usually specifies a respective “system identification code” (SID) and “network identification code” (NID), and each CDMA base station broadcasts its SID and NID in system overhead messages. Thus, when a WCD detects an allowed SID-and-NID combination, the mobile station may connect to the identified system.